In communication systems, radar, direction finding and other broadband multifunction systems having limited aperture space, it is often desirable to efficiently couple a radio frequency transmitter and receiver to an antenna having an array of broadband dual polarized radiator elements.
Conventional broadband phased array radiators generally suffer from significant polarization degradation at large scan angles in the diagonal scan planes. This limitation can force a polarization weighting network to heavily weight a single polarization. Such weighting results in the transmit array having poor antenna radiation efficiency because the unweighted polarization signal must supply most of the antenna Effective Isotropic Radiated Power (EIRP) of the transmitted signal.
Conventional broadband phased array radiators generally use a simple, but asymmetrical feed. Since a conventional broadband radiator is capable of supporting a relatively large set of higher-order propagation modes, the feed region acts as the launcher for these high-order propagation mode signals. The feed is essentially the mode selector or filter. A physical asymmetry in the feed region produces asymmetry in the orientation of launched fields and higher-order modes are excited. Those modes then propagate to the aperture. The higher-order modes cause problems in the radiator performance. The field at the aperture is the superposition of multiple excited modes, and since higher-order modes propagate at differing phase velocities, sharp deviations from uniform magnitude and phase in the unit cell fields result. The fundamental mode aperture excitation is relatively simple, usually resulting from the TE01 mode, with a cosine distribution in the E-plane and uniform field in the H-plane. Significant deviations from the fundamental mode result from the excited higher-order modes, and the higher order modes are responsible for a total mismatch (referred to as a scan blindness or resonance) at certain scan angles and frequencies.
Another effect produced by the presence of higher-order mode propagation in asymmetrically-fed wideband radiators is cross-polarization. Particularly in the diagonal planes, many higher-order modes include an asymmetry that excites the cross-polarized field, which is corrected with an unbalanced weighting in the antenna polarization weighting network resulting in low array transmit power efficiency.
Conventional broadband radiators not only employ an asymmetric feed, but also have offset phase centers which, in dual polarization operation, produce phase errors that cannot be corrected with phase and amplitude compensation over wide instantaneous bandwidths. An array with coincident phase centers eliminates these errors since the phase center for both polarizations is in the center of the unit cell.
U.S. Pat. No. 7,180,457, which is incorporated herein by reference, discloses a prior art electrically short crossed notch (ESCN) radiator in FIG. 1A and a prior art feed circuit in FIG. 1B. The ESCN uses balanced symmetry throughout the unit cell in order to provide superior cross polarization isolation over a 3:1 operating band and a 60 degree conical field of view. A microstrip distribution circuit is backed by a cavity designed to cut off higher order modes capable of launching cross-polarized fields.
FIG. 1A shows the '457 prior art coincident phase center broadband antenna 10 having a wide frequency band, e.g., 3-to-1, with good polarization purity. The antenna 10 includes a cavity plate 12 and an array of notch antenna elements generally denoted 14. Taking a unit cell 14a as representative of each of the unit cells 14, unit cell 14a is provided from four fin-shaped members 16a, 16b, 18a, 18b. Fin-shaped members 16a, 16b, 18a, 18b are disposed on a feed structure. By disposing the members 16a, 16b orthogonal to members 18a, 18b, each unit cell is responsive to orthogonally directed electric field polarizations. That is, by disposing one set of members (e.g. members 16a, 16b) in one polarization direction and disposing a second set of members (e.g. members 18a, 18b) in the orthogonal polarization direction, an antenna that is responsive to signals having any polarization is provided.
In one embodiment, to facilitate the manufacturing process, at least some of the fin-shaped members 16a and 16b can be manufactured as “back-to-back” fin-shaped members as illustrated by member 22. Likewise, the fin-shaped members 18a and 18b can also be manufactured as “back-to-back” fin shaped members as illustrated by member 23. Thus, as can be seen in unit cells 14k and 14k′, each half of a back-to-back fin-shaped member forms a portion of two different notch elements.
FIG. 1B shows an exploded view of the prior art '457 ESCN raised pyramidal feed. A radiator feed circuit 50 is coupled to a bracket 52 with a bond film 54 therebetween. Balun assemblies 58 in the assembly contribute significant cost and part count to manufacture. Output lines 60, grounding gasket 62, and conductive bond films 64 complete the assembly. The microstrip circuit is a molded piece with four legs with opposing legs fed 180 degrees out of phase so that the signals cancel in the throat region of the radiator, launching an odd-mode field between the tapered fins.
While known ESCN designs may provide excellent cross polarization and matching throughout the scan volume, the balun and feed structure have a relatively high part count and a complex and costly assembly process.